The present invention relates to field of integrated circuits and, more particularly, to output power stages.
The invention is directed to a current controlled output stage for driving a load in a voltage mode under normal conditions and in a current mode when the load requires a particularly high current. An output stage of the prior art is illustrated in FIG. 1, in which the load is normally driven in the voltage mode (Vb=Va). When the current absorbed by the load becomes excessive, the driving is automatically controlled in the current mode (I0=K*In).
In certain instances, the output stage may be used to drive a load with a controlled current, i.e., a smaller current than that which could be absorbed by the external load if the delivered current was not otherwise specifically controlled. Furthermore, it is often necessary to provide a full output voltage swing and the ability to meet output voltage constraints both below ground and above supply voltages due to particular operating conditions or noise induced by the load.
As a result of this need for an extended range of operating voltages (e.g., in the car radio segment), it is necessary to provide driving stages with a low voltage drop. For this purpose, a circuit such as the one illustrated in FIG. 2 would traditionally be used, for example. Those of skill in the art will appreciate that voltage mode driving (from node A to node B) is provided by the illustrated circuit, which automatically becomes current mode driving whenever the load requires a current greater than Hfe*Iin (where Hfe is the current gain of the transistor T1). The input variables of the circuit are the voltage provided at the node A by a circuit proceeding the output stage (not shown) and the current Iin, which is provided by a circuit preceding the output stage (also not shown).
The above approach is disadvantageous because the output current depends to a significant extent upon the parameters of the manufacturing process and temperature, as well as the level of the signal itself. Furthermore, this relationship is not of a linear nature because of the variability of the Hfe parameter of the transistor T1, which depends upon the collector current. The output stage illustrated in FIG. 2 may develop output voltages below ground and above the supply voltage. It is only able to drive loads connected to ground, and to deliver and absorb current it must be associated with a corresponding dual structure.
An object of the present invention is to provide an improved driving method and an associated output stage that provides low voltage drop and a wide range of operating voltage characteristics (from below ground values to above supply values), improved current control with relatively small input currents, and which is less affected by processing and temperature variations than the prior art, all irrespective of the level of the input voltage signal.
According to the present invention, a method of voltage driving with controlled current includes providing a negative feedback of an output current measured on a collector of a transistor of the output stage, comparing the current measured at the collector with the input current, and providing the difference therebetween at the base of the transistor to provide the voltage driving.
The collector is coupled to the input of a feedback current mirror, which in turn is coupled to a supply rail of opposite sign than a rail to which the load is coupled. The output of the feedback current mirror is coupled to a first node at which the input current and the current of the collector are compared. This comparison results in a current difference that is forced, by way of a first current mirror coupled to a diode, onto a second node at which the input voltage signal of the output stage is produced.